cwtext converts ASCII streams in stdin to ASCII Morse code (dotscii) or to audio suitable for >/dev/dsp on Linux and other platforms. It also has a collection of tools for pulling info from the Internet and presenting it in Morse code.

Nyquist is an elegant and powerful language for
sound synthesis and music composition. Unlike
score languages that tend to deal only with
events, or signal processing languages that tend
to deal only with signals and synthesis, it
handles both in a single integrated system. It is
also flexible and easy to use because it is based
on an interactive Lisp interpreter. You can design
instruments by combining functions (much as you
would using the orchestra languages of Music V,
cmusic, or Csound). You can call upon these
instruments and generate a sound just by typing a
simple expression. You can combine simple
expressions into complex ones to create a whole
composition. It runs under any Unix environment,
MacOS, Windows 95, and Windows NT, and it
produces sound files as output (or direct audio
output under Windows).

The CMU Midi Toolkit (CMT) is a collection of software
for writing interactive MIDI software in C. It includes a
number of handy utilities along with an application
"shell" that provides timing, scheduling, and MIDI
interfaces that are portable across DOS, Mac, SGI, and
Amiga platforms.

Signal Applications To Audio Networks is a program
aimed at different signal processing tasks. The
primary type of signals are audio signals. The
user describes networks of different modules using
a special language, and the modules are
dynamically loaded libraries. The system can be
used to create music from scratch, or to process
already created audio files, and depending on the
complexity of the network versus the capacity of
the host machine, everything can be done in "real
time".

Audiality is a highly scalable and portable audio engine and
synthesizer. MIDI files are used in combination with scripting and
modular synthesis, to minimize file sizes and maximize flexibility.
Audiality can be used as a music and sound effects player in
multimedia productions, or as a realtime MIDI synthesizer.

The Audio* Music Synthesis Project is music
creation software. Currently it features Mojo
Synth which is a polyphonic analog modelling synth
that has a 64 step sequencer and can also be
played from a MIDI keyboard. It attempts to
emulate classic analog sounds from Roland M-100,
Korg MS-20, and Mini Moog synthesizers.

SynthTest is a somewhat simple application for OS X that allows you to pick a MIDI source, a MusicDevice AudioUnit, and an Effect or MusicEffect AudioUnit to play with. You can open an edit window for both the synth and effect. If they do not have a custom edit GUI, the generic editor view will be used. SynthTest has a reference implementation of preset support for Audio Units. It is primarily intended to be used by Audio Unit developers for testing purposes (the source is available), although it can be great fun to play with all by itself. It is complete enough to be used for live performances.

Javasynth is a Java-based system for creating
(rather experimental) modular audio systems
called sound modules. This allows
synthesizers of some kind to be assembled in
a modular fashion using oscillators, amplifiers,
envelopes, filters, and other components
familiar to those that have worked with
hardware synthesizers. It includes a
separated synth engine and a Swing user
interface for building sound modules.

GtkGEP turns your computer into a realtime effects processor. You can
plug your guitar into the computer and play with cool distortion
effects, for example. It has a modular plugin structure, with standard
plugins including distortion, overdrive, delay, reverb, equalizers,
and a flanger. It works in 16-bit resolution, in mono mode, and with
frequencies from 11khz to 44khz. The sound quality is very good.